Process control systems are widely used in factories and/or plants in which products are manufactured or processes are controlled (e.g., chemical manufacturing, power plant control, etc.). Process control systems are also used in the harvesting of natural resources such as, for example, oil and gas drilling and handling processes, etc. In fact, virtually any manufacturing process, resource harvesting process, etc. can be automated through the application of one or more process control systems. It is believed the process control systems will eventually be used more extensively in agriculture as well.
Distributed process control systems, like those used in chemical, petroleum or other processes, typically include one or more centralized or decentralized process controllers communicatively coupled to at least one host or operator workstation and to one or more process control and instrumentation devices, such as field devices, via analog, digital or combined analog/digital buses or via a wireless communication link or network. A process controller (sometimes referred to as a “controller”), which is typically located within the plant or other industrial environment, receives signals (sometimes referred to as “control inputs”) indicative of process measurements and uses the information carried by these signals to implement control routines that cause the controller to generate control signals (sometimes referred to as “control outputs”) based on the control inputs and the internal logic of the control routines. The controllers send the generated control signals over buses or other communication links to control operation of field devices. In some instances, the controllers may coordinate with control routines implemented by smart field devices, such as Highway Addressable Remote Transmitter (HART®), WirelessHART®, and FOUNDATION® Fieldbus (sometimes just called “Fieldbus”) field devices. Moreover, in many cases, there may be plant or other industrial equipment that operates in the plant or other industrial setting to perform some function that is not under direct control of the process controller, such as vibration detection equipment, rotating equipment, electrical power generating equipment, etc.
Field devices that are typically associated with the controller, which may be, for example valves, valve positioners, switches, transmitters, and sensors (e.g., temperature, pressure and flow rate sensors), are located within the process environment and generally perform physical or process control functions. For example, a valve may open or close in response to a control output received from a controller, or may transmit to a controller a measurement of a process parameter so that the controller can utilize the measurement as a control input. Smart field devices, such as field devices conforming to the well-known Fieldbus protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within a controller. Field devices may be configured to communicate with controllers and/or other field devices according to various communication protocols. For example, a plant may include traditional analog 4-20 mA field devices, HART® field devices, Fieldbus field devices, and/or other types of field devices.
The process controllers receive signals indicative of process measurements or process variables made by or associated with the field devices and/or other information pertaining to the field devices, and execute a controller application that runs, for example, different control modules that make process control decisions, generate control signals based on the received information, and coordinate with the control modules or blocks being performed in the field devices. The control modules in the controller send the control signals over the communication lines or links to the field devices to thereby control the operation of at least a portion of the process plant or system.
Information from the field devices and the controller is usually made available over a data highway to one or more other hardware devices, such as operator workstations, personal computers, or computing devices, data historians, report generators, centralized databases, or other centralized administrative computing devices that are typically, but not always, placed in control rooms or other locations away from the harsher plant environment. Each of these hardware devices typically, though not always, is centralized across the process plant or across a portion of the process plant. These hardware devices run applications that may, for example, enable an operator to perform functions with respect to controlling a process and/or operating the process plant, such as changing settings of the process control routine, modifying the operation of the control modules within the controllers or the field devices, viewing the current state of the process, viewing alarms generated by field devices and controllers, simulating the operation of the process for the purpose of training personnel or testing the process control software, keeping and updating a configuration database, etc. The data highway utilized by the hardware devices, controllers, and field devices may include a wired communication path, a wireless communication path, or a combination of wired and wireless communication paths.
As an example, the DeltaV™ control system, sold by Emerson Process Management, includes multiple applications stored within and executed by different devices located at diverse places within a process plant. A configuration application, which resides in one or more operator workstations or computing devices, enables users to create or change process control modules and download these process control modules via a data highway to dedicated distributed controllers. Typically, these control modules are made up of communicatively interconnected function blocks, which perform functions within the control scheme based on inputs thereto and which provide outputs to other function blocks within the control scheme. The configuration application may also allow a configuration designer to create or change operator interfaces which are used by a viewing application to display data to an operator and to enable the operator to change settings, such as set points, within the process control routines. Each dedicated controller and, in some cases, one or more field devices, stores and executes a respective controller application that runs the control modules assigned and downloaded thereto to implement actual process control functionality. The viewing applications, which may be executed on one or more operator workstations (or on one or more remote computing devices in communicative connection with the operator workstations and the data highway), receive data from the controller application via the data highway and display this data to process control system designers, operators, or users using the operator interfaces, and may provide any of a number of different views, such as an operator's view, an engineer's view, a technician's view, etc. A data historian application is typically stored in and executed by a data historian device that collects and stores some or all of the data provided across the data highway while a configuration database application may run in a still further computer attached to the data highway to store the current process control routine configuration and data associated therewith. Alternatively, the configuration database may be located in the same workstation as the configuration application.
As noted above, operator display applications are typically implemented on a system wide basis in one or more of the workstations and provide displays to the operator or maintenance persons regarding the operating state of the control system or the devices within the plant. Typically, these displays take the form of alarming displays that receive alarms generated by controllers or devices within the process plant, control displays indicating the operating state of the controllers and other devices within the process plant, maintenance displays indicating the operating state of the devices within the process plant, etc. These displays are generally configured to display, in known manners, information or data received from the process control modules or the devices within the process plant. In some known systems, displays have a graphic associated with a physical or logical element that is communicatively tied to the physical or logical element to receive data about the physical or logical element. The graphic may be changed on the display screen based on the received data to illustrate, for example, that a tank is half full, to illustrate the flow measured by a flow sensor, etc.
Traditional analog 4-20 mA field devices communicate with a controller via a two-wire communication link (sometimes called a “loop” or “current loop”) configured to carry a 4-20 mA DC signal indicative of a measurement or control command. For example, a level transmitter may sense a tank level and transmit via the loop a current signal corresponding to that measurement (e.g., a 4 mA signal for 0% full, a 12 mA signal for 50% full, and a 20 mA signal for 100% full). The controller receives the current signal, determines the tank level measurement based on the current signal, and takes some action based on the tank level measurement (e.g., opening or closing an inlet valve). Analog 4-20 mA field devices typically come in two varieties including four-wire field devices and two-wire field devices. A four-wire field device typically relies on a first set of wires (i.e., the loop) for communication, and a second set of wires for power. A two-wire field device relies on the loop for both communication and power. These two-wire field devices may be called “loop powered” field devices.
Process plants often implement traditional 4-20 mA systems due to the simplicity and effectiveness of the design. Unfortunately, traditional 4-20 mA current loops only transmit one process signal at a time. Thus, a set-up including a control valve and a flow transmitter on a pipe carrying material may require three separate current loops: one for carrying a 4-20 mA signal indicative of a control command for the valve (e.g., to move the valve to 60% open); a second for carrying a 4-20 mA signal indicative of the valve's actual position (e.g., so that the controller knows the degree to which the valve has responded to control commands); and a third for carrying a 4-20 mA signal indicative of a measured flow. As a result, a traditional 4-20 mA set-up in a plant having a large number of field devices may require extensive wiring, which can be costly and can lead to complexity when setting up and maintaining the communication system.
More recently, the process control industry has moved to implement digital communications within the process control environment. For example, the HART® protocol uses the loop DC magnitude to send and receive analog signals, but also superimposes an AC digital carrier signal on the DC signal to enable two-way field communications with smart field instruments. As another example, the Fieldbus protocol provides all-digital communications on a two-wire bus (sometimes called a “segment” or “Fieldbus segment”). This two-wire Fieldbus segment can be coupled to multiple field devices to provide power to the multiple field devices (via a DC voltage available on the segment) and to enable communication by the field devices (via an AC digital communication signal superimposed on the DC power supply voltage). Generally speaking, because the connected field devices use the same segment for communication and are connected in parallel, only one field device can transmit a message at any given time over the segment. Accordingly, communication on a segment is coordinated by a device designated as a link active scheduler (LAS). The LAS is responsible for passing a token between field devices connected to the segment. Only the device with the token may communicate over the segment at a particular time.
These digital communication protocols generally enable more field devices to be connected to a particular communication link, support more and faster communications between the field devices and the controller, and/or allow field devices to send more and different types of information (such as information pertaining to the status and configuration of the field device itself) to the process controller and other devices in or connected to the control network. Furthermore, these standard digital protocols enable field devices made by different manufacturers to be used together within the same process control network.
The various devices within the process plant may be interconnected in physical and/or logical groups to create a logical process, such as a control loop. Likewise, a control loop may be interconnected with other control loops and/or devices to create sub-units. A sub-unit may be interconnected with other sub-units to create a unit, which in turn, may be interconnected with other units to create an area. Process plants generally include interconnected areas, and business entities generally include process plants which may be interconnected. As a result, a process plant includes numerous levels of hierarchy having interconnected assets, and a business enterprise may include interconnected process plants. In other words, assets related to a process plant, or process plants themselves, may be grouped together to form assets at higher levels.
Thus, one particularly important aspect of process control system design involves the manner in which field devices are communicatively coupled to each other, to controllers and to other systems or devices within a process control system or a process plant. In general, the various communication channels, links and paths that enable the field devices to function within the process control system are commonly collectively referred to as an input/output (I/O) communication network.
The communication network topology and physical connections or paths used to implement an I/O communication network can have a substantial impact on the robustness or integrity of field device communications, particularly when the I/O communications network is subjected to environmental factors or conditions associated with the process control system. For example, many industrial control applications subject field devices and their associated I/O communication networks to harsh physical environments (e.g., high, low or highly variable ambient temperatures, vibrations, corrosive gases or liquids, etc.), difficult electrical environments (e.g., high noise environments, poor power quality, transient voltages, etc.), etc. In any case, environmental factors can compromise the integrity of communications between one or more field devices, controllers, etc. In some cases, such compromised communications could prevent the process control system from carrying out its control routines in an effective or proper manner, which could result in reduced process control system efficiency and/or profitability, excessive wear or damage to equipment, dangerous conditions that could damage or destroy equipment, building structures, the environment and/or people, etc.
In order to minimize the effect of environmental factors and to assure a consistent communication path, I/O communication networks used in process control systems have historically been hardwired networks, with the wires being encased in environmentally protected materials such as insulation, shielding and conduit. Also, the field devices within these process control systems have typically been communicatively coupled to controllers, workstations, and other process control system components using a hardwired hierarchical topology in which non-smart field devices are directly coupled to controllers using analog interfaces such as, for example, 4-20 mA, 0-10 VDC, etc. hardwired interfaces or I/O boards. Smart field devices, such as Fieldbus devices, are also coupled via hardwired digital data busses, which are coupled to controllers via smart field device interfaces.
While hardwired I/O communication networks can initially provide a robust I/O communication network, their robustness can be seriously degraded over time as a result of environmental stresses (e.g., corrosive gases or liquids, vibration, humidity, etc.). For example, contact resistances associated with the I/O communication network wiring may increase substantially due to corrosion, oxidation and the like. In addition, wiring insulation and/or shielding may degrade or fail, thereby creating a condition under which environmental electrical interference or noise can more easily corrupt the signals transmitted via the I/O communication network wires. In some cases, failed insulation may result in a short circuit condition that results in a complete failure of the associated I/O communication wires.
Additionally, hardwired I/O communication networks are typically expensive to install, particularly in cases where the I/O communication network is associated with a large industrial plant or facility that is distributed over a relatively large geographic area, for example, an oil refinery or chemical plant that consumes several acres of land. In many instances, the wiring associated with the I/O communication network must span long distances and/or go through, under or around many structures (e.g., walls, buildings, equipment, etc.) Such long wiring runs typically involve substantial amounts of labor, material and expense. Further, such long wiring runs are especially susceptible to signal degradation due to wiring impedances and coupled electrical interference, both of which can result in unreliable communications.
Moreover, such hardwired I/O communication networks are generally difficult to reconfigure when modifications or updates are needed. Adding a new field device typically requires the installation of wires between the new field device and a controller. Retrofitting a process plant in this manner may be very difficult and expensive due to the long wiring runs and space constraints that are often found in older process control plants and/or systems. High wire counts within conduits, equipment and/or structures interposing along available wiring paths, etc., may significantly increase the difficulty associated with retrofitting or adding field devices to an existing system. Exchanging an existing field device with a new device having different field wiring requirements may present the same difficulties in the case where more and/or different wires have to be installed to accommodate the new device. Such modifications may often result in significant plant downtime.
Wireless I/O communication networks have been used to alleviate some of the difficulties associated with hardwired I/O networks, and to alleviate the costs involved in deploying sensors and actuators within the process control system. Wireless I/O communication networks have also been suggested for process control systems and portions thereof that are relatively inaccessible or inhospitable for hardwired I/O communication networks. For example, Shepard et al., U.S. Pat. No. 7,436,797 entitled “Wireless Architecture And Support For Process Control Systems” and patented Oct. 14, 2008, the content of which is expressly incorporated by reference herein, discloses that relatively inexpensive wireless mesh networks may be deployed within a process control system, either alone or in combination with point-to-point communications, to produce a robust wireless communication network that can be easily set up, configured, changed and monitored, to thereby make the wireless communication network more robust, less expensive and more reliable.
Wireless mesh communication networks (or mesh networking topology) utilize multiple nodes, each of which may serve not only as a client to receive and send its own data, but also as a repeater or relay to propagate data through the network to other nodes. Each node is connected to another neighboring node, and preferably to multiple neighboring nodes, each of which may be connected to additional neighboring nodes. The result is a network of nodes that provides multiple paths of communication from one node to another through the network, thereby creating a relatively inexpensive, robust network that allows for continuous connections and reconfigurations even when communication paths are broken or blocked.
In a wireless mesh network, each device (node) may connect to a gateway via direct wireless connection or indirectly via a connection through a neighboring device. Each device has a signal strength that generally correlates to the physical proximity of the device to the wireless gateway or to a neighboring device. In cases where no direct connection to the wireless gateway is available, each device connects to the gateway through another peer device that has a connection to the gateway or to another device. The number of relay nodes used to chain together a connection of another node to the gateway is known as the number of hops in a connection path. Each device uses the connection path, and the order in which the device-to-device connections are established is known as the communication route.
One such wireless mesh network used in process control is the WirelessHART® mesh network (such as the WirelessHART® mesh network described by the international standard IEC 62591) developed by the HART Communication Foundation. Generally speaking, a WirelessHART® mesh network is a multi-hop communication network having a gateway and multiple WirelessHART® devices (wireless nodes). The network is organized in a mesh topology and each device is capable of routing messages for other devices in order to relay data to and from the gateway. WirelessHART® devices are capable of self-diagnostics and generate their own alerts and wireless communication statistics. Additional diagnostics also exist to identify when a wireless node is a pinch point. A pinch point is a wireless node whose failure would result in at least one other wireless node no longer having a communication path to the gateway of the wireless mesh network. Given a wireless mesh network, data about the WirelessHART® devices and communication connectivity conditions, such as battery condition, poor communications (e.g., low communication path stability) and pinch point devices may be collected and analyzed. However, although such diagnostics provide information on the existing connectivity conditions of the devices and the network, there is a lack of diagnostics on the potential impact a connectivity condition may have on the rest of the wireless mesh communication network.